1. Field of the Invention
The invention relates to a cellular phone, and more particularly to optimization in antenna performance in a cellular phone covering a plurality of different frequency areas.
2. Description of the Related Art
It is known that an antenna irradiation pattern indicating antenna characteristic of a cellular phone varies in independence on a length of a ground area of an antenna matching circuit which electrically connects an antenna and a high-frequency circuit to each other. Herein, a ground area indicates a ground of a printed substrate or a die-cast body, for instance. If such a ground area does not have an optimal length, a lot of poles called a null point appear in an antenna irradiation pattern. Herein, a null point means a point at which an antenna gain falls down.
Some cellular phones such as PDC (personal digital cellular) or W-CDMA (Wideband-Code Division Multiple Access) are designed to be able to cover a plurality of frequency bands. If such cellular phones have to cover two frequency bands quite different from each other, for instance, 800 MHz band and 2 GHz band, it would be quite difficult or almost impossible to accomplish antenna characteristic by which null points are not generated, through a single ground length.
If a lot of null points are generated in an antenna irradiation pattern, an antenna gain much fluctuates in dependence on a position of a cellular phone, and hence, a level of a signal received from a base station and a level of a signal transmitted to a base station much fluctuate, resulting in unstable radio-signal transmission condition.
Japanese Patent Application Publication No. 7-283631 has suggested an antenna unit including a radio-signal transceiver arranged in a body, an antenna device extendable from and retractable into the radio-signal transceiver, and a coil which operates as a matching circuit when the antenna device is extended, and operates as a helical antenna when the antenna device is retracted.
Japanese Patent Application Publication No. 2001-339474 has suggested a foldable type cellular phone including a main body, and an antenna matching circuit. The main body is comprised of a first body, and a second body designed pivotable relative to the first body. The antenna matching circuit is designed to have different characteristics in accordance with a length of the cellular phone in a folded state and a length of the cellular phone in a non-folded state.